A case for 6+ won't fit on 7+, but a case for 7+ will fit on 6+. There is just two small issues. First, the cut out for rear camera on a 7+ case will be touching the rear camera lens of a 6+, though it doesn't obstruct the view so not a real problem. Second, if the bottom of 7+ case has 3 cut outs for the speakers and lightning port then the left cutout will cover the hole for 3.5mm jack. Now, you can see the hole through this cutout but it's too small for 3.5mm jack to pass through. So if there happens to be a 7+ case which is so beautiful that you want to put it on your 6+ then you will have to make this cutout slightly bigger (a 4mm drill bit maybe; or brute force hot needle through plastic). Finally, there are some 7+ cases which open from bottom or have a single large cut out. Haven't tried any of those on my 6+, so not sure whether there is room for 3.5mm jack to be plugged. I just upgraded from 6+ to 7+ but keeping my old 6+ as well. Good 6+ cases are harder to find whereas I got a very nice combo deal for 7+ case with a tempered glass screen protector for only 20$ ( nearly what you will pay for just a good tempered glass screen protector), so I bought two of them and will just drill a hole on left cutout to be perfect for 6+.

They are identical dimensions. When shopping for a case, however, be sure to notice that the camera cutout on a 7 Pkus case is going to have to be wider to allow for the dual cameras, and the bottom of the phone is also going to be built differently (stereo speakers, no headphone jack).